1. Field of the Invention
The present invention relates to an ultrasonic Doppler diagnostic apparatus, more particularly to an improved ultrasonic Doppler diagnostic apparatus capable of accurately detecting or measuring the velocity of movement of moving reflective members, e.g. blood flow within the heart or within a coronary artery, in a subject under observation.
2. Description of the Prior Art
The ultrasonic pulsed Doppler method has been practically applied to the measurement of the velocity of movement of moving reflective members within a subject under observation, specifically to the measurement of the velocity of movement of blood flow in the body organs such as the heart, of blood flow in the circulatory organs and blood vessels, of other body fluids, and of the cardiac muscles. The velocity of movement of the moving reflective member is electrically detected from the frequency shift of an echo reflected from the moving reflective member within the subject. More specifically, an ultrasonic pulsed wave of a given fixed pulse repetition frequency is transmitted into a subject, the echo from a moving reflective member in the subject is received, the distance to the moving reflective member is determined from the time required for the ultrasonic pulsed wave to travel to and from the moving reflective member, and the velocity of the moving reflective member is determined by calculating the frequency shift (resulting from the Doppler effect) in the frequency of the received echo signal.
However, the conventional ultrasonic Doppler diagnostic apparatus has a problem in that when a clutter signal (i.e. a signal reflected from a slow moving blood vessel wall, heart wall or cardiac muscle) which has a larger amplitude than the Doppler signal (i.e. a signal including the Doppler shift frequency) representing the velocity of movement of the blood flow or the like gets mixed with the Doppler signal, the presence of the clutter signal hinders the detection and measurement of the velocity of movement of the moving reflective member.
Conventionally, the strength of clutter signals of this type has been reduced by using a filter for eliminating low frequency components. For example, in a color flow mapping system which provides a real-time B-mode display of the velocity distribution of moving reflective members, while there is used a delay-line canceler or other type filter with comb-like frequency characteristics, it is necessary for effective elimination of the clutter signal component to use a filter with still sharper cutoff frequency characteristics. The problem is, however, that sharper filters have more complex structures.
Moreover, it is known that the use of a filter with sharp cutoff frequency characteristics results in a longer response time because of the numerous delay-line stages involved, which in turn degrades the real-time characteristics of the detected image.
On the other hand, the conventional filter having a short response time is incapable of adequately suppressing the clutter signals and thus is incapable of overcoming the problem of reduced measurement precision.
Further, since the amplitude of the clutter signal is greater than the amplitude of the blood flow signal, even if the clutter signal is removed by a filter to some extent, the remaining portion thereof will still prevent the detection of some moving reflective members. In this regard, consider the case of a coronary artery running along the heart wall. Since the coronary artery itself moves together with the movement of the heart wall, the signal representing the flow of blood within the coronary artery will be hidden within a clutter signal caused by the movement of the heart wall. A similar situation arises in the case of blood flow within the heart in the vicinity of the heart wall, and in fact it has not been possible to measure the velocity of such blood flow accurately with the conventional apparatus.